Large-aperture and high-resolution laser Thomson scattering multi-grating spectrometer

Abstract

The invention discloses a novel large-aperture and high-resolution multi-grating spectrometer applied to laser Thomson scattering, which adopts a mixed optical structure of a large-aperture off-axis parabolic mirror and a long-focus lens to jointly realize collimation and focusing of an internal light path of the multi-grating spectrometer. According to the specific technical scheme, the device comprises a first spectrograph, an optical mask, a second spectrograph and a third spectrograph. The first spectrograph comprises an entrance slit, an off-axis parabolic mirror, a plane reflection type diffraction grating and a lens; wherein the entrance slit is a light inlet of the multi-grating spectrometer and is located on a focus of the off-axis parabolic mirror, the off-axis parabolic mirror collimates scattered light from the entrance slit into parallel light, the plane reflection type diffraction grating disperses and splits the parallel light, and the lens focuses the scattered light after dispersion to the focus. Compared with a common laser Thomson scattering multi-grating spectrometer in which focusing of a focusing lens is limited due to the fact that lenses with the same parameters are adopted in a focusing element and a collimating element, the long-focus lens is adopted as a light beam focusing element, and the spectral resolution of the spectrometer is improved.

Description

Technical Field

[0001] The invention relates to the research field of low-temperature plasma laser spectrum diagnosis, and in particular to a new type of large-aperture and high-resolution multi-grating spectrometer applied to laser Thomson scattering. Background Art

[0002] In low-temperature plasma, free electrons are important energy transfer particles, which can profoundly affect the distribution of other heavy particles (atoms, molecules, ions, etc.) through collision reactions (such as ionization, excitation, dissociation, recombination, deexcitation, etc.). Accurate diagnosis of plasma electron density and electron temperature helps to deeply understand the complex physical mechanisms inside the plasma. Among the commonly used plasma diagnostic methods, laser Thomson scattering technology has the advantages of not interfering with the operating state of the plasma, high temporal and spatial resolution, and independence from whether the plasma is in a local thermodynamic equilibrium state. It has been recognized as one of the most accurate methods for diagnosing electron density and electron temperature. However, in the diagnostic application of low-temperature plasma, the low differential scattering cross section (7.9×10 -30 m 2 ) causes the laser Thomson scattering signal to be very weak, which is easily annihilated by strong signals of the central wavelength such as stray light and Rayleigh scattering. Therefore, how to suppress strong signals such as stray light and Rayleigh scattering is the key to obtaining Thomson scattering signals. Multi-grating spectrometers can effectively suppress strong signals such as stray light and Rayleigh scattering through physical shielding, and the suppression ratio can reach 10 -10 , has been widely used in laser Thomson scattering diagnostic systems. However, in multi-grating spectrometers, optical lenses are often used as light path collimation and focusing elements. Since the lens aperture is inversely proportional to the focal length, and the resolution is proportional to the focal length of the lens, it is difficult for a lens-based multi-grating spectrometer to have both large aperture and high resolution. Summary of the invention

[0003] In view of the problems existing in the prior art, the present invention discloses a novel multi-grating spectrometer with large aperture and high resolution applied to laser Thomson scattering, which adopts a hybrid optical structure of large aperture off-axis parabolic reflector and telephoto lens to realize the collimation and focusing of the internal optical path of the multi-grating spectrometer. The specific technical scheme includes: a first spectrometer, an optical mask, a second spectrometer and a third spectrometer;

[0004] a first spectrometer, an optical mask, a second spectrometer, and a third spectrometer;

[0005] The first spectrometer comprises an incident slit, a first off-axis parabolic reflector, a first plane reflective diffraction grating and a first lens; wherein the incident slit is a light entrance of the multi-grating spectrometer and is located at the focus of the first off-axis parabolic reflector, the first off-axis parabolic reflector collimates scattered light from the incident slit into parallel light, the first plane reflective diffraction grating disperses and splits the parallel light, and the first lens focuses the dispersed scattered light to the focus;

[0006] The optical mask is located at the center of the focal plane of the first lens in the first spectrometer to shield the stray light of the central wavelength and the strong signal of Rayleigh scattering;

[0007] The second spectrometer comprises a second lens, a second plane reflective diffraction grating and a second off-axis parabolic reflector; the second lens collimates the scattered light not shielded by the optical mask into parallel light, the second plane reflective diffraction grating reversely disperses the parallel light, and the second off-axis parabolic reflector focuses the reversely dispersed light to a focal point;

[0008] The third spectrometer includes a middle slit, a third off-axis parabolic reflector, a third plane reflective diffraction grating, a third lens and a detector. The middle slit is located at the focal position of the off-axis parabolic reflector of the second spectrometer and serves as a light inlet of the third spectrometer while shielding stray light that does not propagate along the optical path. The third off-axis parabolic reflector collimates scattered light from the middle slit into parallel light. The third plane reflective diffraction grating disperses and splits the parallel light. The third lens focuses the dispersed parallel light to a focal point. The detector is located at the focal position of the third lens and detects scattered light signals.

[0009] The first off-axis parabolic reflector, the second off-axis parabolic reflector and the third off-axis parabolic reflector are short-focus large-aperture off-axis parabolic reflectors.

[0010] The first lens, the second lens and the third lens are telephoto lenses.

[0011] The incident slit is a conventional slit, and the middle slit is a slit with adjustable slit width, which is used to adjust the spectral resolution of the multi-grating spectrometer and shield the stray light inside the spectrometer that does not propagate along the optical path.

[0012] Due to the adoption of the above technical scheme, a large-aperture and high-resolution laser Thomson scattering multi-grating spectrometer provided by the present invention adopts a short-focus large-aperture off-axis parabolic reflector as a beam collimation element for coupling the incident slit and the first spectrometer, the intermediate slit and the third spectrometer. Compared with the lens having a larger thickness and more serious aberration when the lens is short-focus and large-aperture, the short-focus large-aperture off-axis parabolic reflector is not affected by thickness, and at the same time, the spherical aberration, astigmatism and chromatic aberration are zero, and the aberration is small. Therefore, as many scattered photons as possible can be collected while ensuring imaging treatment, thereby improving the light-gathering ability of the spectrometer.

[0013] The present invention proposes to use a telephoto lens as a beam focusing element. Compared with a common laser Thomson scattering multi-grating spectrometer, since both the focusing element and the collimating element use lenses with the same parameters, the focusing of the focusing lens is limited, thereby improving the spectral resolution of the spectrometer.

[0014] The present invention proposes a multi-grating spectrometer that combines a long-focus lens and a short-focus large-aperture off-axis parabolic reflector, which solves the technical difficulty that a common laser Thomson scattering multi-grating spectrometer is difficult to have both a large aperture and a high resolution, and improves the detection sensitivity of the multi-grating spectrometer. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0016] Figure 1 The structure diagram of the large aperture and high resolution laser Thomson scattering multi-grating spectrometer of the present invention DETAILED DESCRIPTION

[0017] In order to make the technical solutions and advantages of the present invention more clear, the technical solutions in the embodiments of the present invention are clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention:

[0018] like Figure 1 A large aperture and high resolution laser Thomson scattering multi-grating spectrometer is shown, comprising a first spectrometer, an optical mask 5, a second spectrometer and a third spectrometer.

[0019] Furthermore, the first spectrometer includes an incident slit 1, a first off-axis parabolic reflector 2, a first plane reflective diffraction grating 3 and a first lens 4; wherein the incident slit 1 is the light entrance of the multi-grating spectrometer and is located at the focus of the first off-axis parabolic reflector 2, the first off-axis parabolic reflector 2 collimates the scattered light from the incident slit 1 into parallel light, the first plane reflective diffraction grating 3 disperses the parallel light, and the first lens 4 focuses the dispersed scattered light to the focus. In working state: the Thomson scattered light signal to be measured enters the spectrometer through the incident slit 1, is irradiated onto the first off-axis parabolic reflector 2, and is collimated into a parallel light beam. The short-focus large-aperture off-axis parabolic reflector can receive more scattered photons, thereby improving the light-collecting ability of the multi-grating spectrometer; the parallel light beam is projected onto the plane reflective diffraction grating 3 and is dispersed and split; the dispersed parallel light beam is focused to the focal point by the first lens 4, and light of different wavelengths is distributed at different spatial positions. The first lens 4 adopts a telephoto lens, thereby improving the resolution of the multi-grating spectrometer.

[0020] The optical mask 5 is located at the focal center of the first lens 4 to filter out strong signals of the central wavelength (including stray light and Rayleigh scattering). The optical mask 5 adopts a modular design structure, which can realize rapid disassembly and replacement of optical masks of different widths to meet different measurement requirements.

[0021] The second spectrometer includes a second lens 6, a second plane reflective diffraction grating 7 and a second off-axis parabolic reflector 8. The second lens 6 collimates the scattered light not shielded by the optical mask 5 into parallel light; the second plane reflective diffraction grating 7 reversely disperses the parallel light; and the second off-axis parabolic reflector 8 focuses the reversely dispersed light to a focal point. The second off-axis parabolic reflector 8 adopts a short-focus large-aperture off-axis parabolic reflector, and the second lens 6 adopts a long-focus lens.

[0022] Furthermore, the third spectrometer includes a middle slit 9, a third off-axis parabolic reflector 10, a third plane reflective diffraction grating 11, a third lens 12 and a detector 13. The third off-axis parabolic reflector 10 adopts a short-focus large-aperture off-axis parabolic reflector to improve the light-collecting ability of the multi-grating spectrometer. The third lens 12 adopts a telephoto lens to improve the resolution of the multi-grating spectrometer. The middle slit 9 adopts a slit with adjustable width. The middle slit 9 is located at the focal position of the large-aperture off-axis parabolic reflector of the second spectrometer, serving as the light inlet of the third spectrometer and shielding stray light that does not propagate along the optical path. The third off-axis parabolic reflector 10 collimates the scattered light from the middle slit into parallel light; the third plane reflective diffraction grating 11 disperses and splits the parallel light; the third lens 12 focuses the dispersed parallel light to the focal point; the detector 13 is located at the focal position of the third lens to detect the scattered light signal.

[0023] Furthermore, the incident slit 1 can be a conventional slit, or can be replaced by an optical fiber bundle array according to actual needs. The middle slit 9 is a slit with adjustable slit width, which is used to adjust the spectral resolution of the multi-grating spectrometer and shield the stray light inside the spectrometer that does not propagate along the optical path, so as to prevent the stray light from entering the third spectrometer.

[0024] Furthermore, the first off-axis parabolic reflector 2 and the third off-axis parabolic reflector 10 in the first spectrometer and the third spectrometer are mainly used to collimate the scattered light signals entering the first spectrometer and the third spectrometer through the incident slit and the intermediate slit. The reflector adopts a short-focus and large-aperture design to ensure that as many scattered photons entering the first and third spectrometers as possible are coupled into the optical path, thereby enhancing the light-collecting ability of the spectrometer and improving the detection sensitivity; the second off-axis parabolic reflector 8 in the second spectrometer is mainly used to focus the scattered light compounded by the second plane reflective diffraction grating 7.

[0025] Furthermore, the first plane reflective diffraction grating 3 and the third plane reflective diffraction grating 11 in the first spectrometer and the third spectrometer of the present invention are mainly used for dispersive spectroscopy of the incident light beam; the second plane reflective diffraction grating 7 in the second spectrometer is mainly used for inverse dispersion recombination of the light beam. The blazing wavelength of the grating is close to the wavelength of the incident laser, which improves the efficiency.

[0026] The first lens 4 and the third lens 12 in the first and third spectrometers of the present invention are mainly used to focus the parallel light dispersed by the first plane diffraction reflection grating 3 and the third plane reflection diffraction grating 11, so that the light of different wavelengths is distributed in different spatial positions. The second lens 6 in the second spectrometer is used to collimate the scattered light that is not shielded by the optical mask. The lens adopts a telephoto design to improve the resolution of the multi-grating spectrometer. At the same time, the second lens 6 facilitates the optical mask 5 to more effectively shield the strong signal of the central wavelength.

[0027] Example:

[0028] like Figure 1As shown, the incident slit 1 is located at the focal position of the first off-axis parabolic reflector 2 (focal length of 100 mm, diameter of 100 mm, F number of about F / 1), the Thomson scattered light signal to be measured enters the spectrometer through the incident slit 1 at a certain divergence angle, irradiates the off-axis parabolic reflector 2, and is collimated into a parallel light beam; the parallel light beam is projected onto the diffraction grating 3 (the number of lines is 1800l / mm, the size is 110mm×110mm) at a certain incident angle. The first plane diffraction reflection grating 3 performs dispersion and spectroscopy on the scattered light; the dispersed scattered light beam leaves the first plane diffraction reflection grating 3 at a certain diffraction angle and is focused by the first lens 4 (focal length of 500 mm, diameter of 100 mm) onto the focal plane of the lens 4, and scattered light of different wavelengths is distributed at different spatial positions, wherein strong signals such as stray light and Rayleigh scattering of the central wavelength are distributed at the central position; the optical mask 5 is located at the central position of the focal plane of the first lens 4, shielding the strong signal of the central wavelength (including stray light and Rayleigh scattering); the second lens 6 focuses the unshielded The light is collimated to form a parallel light beam; the parallel light beam is projected onto the second plane reflective diffraction grating 7; the second plane reflective diffraction grating 7 recombines the light beam by inverse dispersion; the recombined light beam is focused by the second off-axis parabolic reflector 8; the middle slit 9 is located at the focal position of the second off-axis parabolic reflector 8; after passing through the middle slit 9, the light beam is collimated into a parallel light beam by the third off-axis parabolic reflector 10 and then dispersed by the third plane reflective diffraction grating 11; the dispersed light beam is focused by the third lens 12 to the detector 13; the detector 13 detects and records the Thomson scattering signal.

[0029] The present invention is not limited to the laser Thomson scattering multi-grating spectrometer described in the above embodiments, wherein changes in parameters (including but not limited to diameter, focal length, grating constant, incident angle, slit width, etc.) of optical elements (including incident slit, intermediate slit, off-axis parabolic reflector, lens, optical mask, etc.) are all within the protection scope of the present invention.

[0030] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A large aperture and high resolution laser Thomson scattering multi-grating spectrometer, characterized in that include: A first spectrometer, an optical mask (5), a second spectrometer and a third spectrometer; The first spectrometer comprises an incident slit (1), a first off-axis parabolic reflector (2), a first plane reflective diffraction grating (3) and a first lens (4); wherein the incident slit (1) is a light entrance of the multi-grating spectrometer and is located at the focus of the first off-axis parabolic reflector (2); the first off-axis parabolic reflector (2) collimates scattered light from the incident slit (1) into parallel light; the first plane reflective diffraction grating (3) disperses and splits the parallel light; and the first lens (4) focuses the dispersed scattered light to the focus; The optical mask (5) is located at the center of the focal plane of the first lens (4) in the first spectrometer to shield stray light and strong Rayleigh scattering signals of the central wavelength; The second spectrometer comprises a second lens (6), a second plane reflective diffraction grating (7) and a second off-axis parabolic reflector (8); the second lens (6) collimates scattered light that is not shielded by the optical mask (5) into parallel light, the second plane reflective diffraction grating (7) reversely disperses the parallel light, and the second off-axis parabolic reflector (8) focuses the reversely dispersed light to a focal point; The third spectrometer comprises a middle slit (9), a third off-axis parabolic reflector (10), a third plane reflective diffraction grating (11), a third lens (12) and a detector (13); the middle slit (9) is located at the focal position of the off-axis parabolic reflector (8) of the second spectrometer and serves as a light inlet of the third spectrometer while shielding stray light that does not propagate along the optical path; the third off-axis parabolic reflector (10) collimates scattered light from the middle slit (9) into parallel light; the third plane reflective diffraction grating (11) disperses and splits the parallel light; the third lens (12) focuses the dispersed parallel light to a focal point; the detector (13) is located at the focal position of the third lens (12) and detects scattered light signals.

2. A large aperture and high resolution laser Thomson scattering multi-grating spectrometer according to claim 1, characterized in that: The first off-axis parabolic reflector (2), the second off-axis parabolic reflector (8) and the third off-axis parabolic reflector (10) are short-focus large-aperture off-axis parabolic reflectors.

3. A large aperture and high resolution laser Thomson scattering multi-grating spectrometer according to claim 1, characterized in that: The first lens (4), the second lens (6) and the third lens (12) are telephoto lenses.

4. A large aperture and high resolution laser Thomson scattering multi-grating spectrometer according to claim 1, characterized in that: The incident slit (1) is a conventional slit, and the middle slit (6) is a slit with adjustable slit width, which is used to adjust the spectral resolution of the multi-grating spectrometer and shield stray light inside the spectrometer that does not propagate along the optical path.

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